Playback modes for groups

Hi there,

I made this little addition to the groups, maybe it can be helpful.
Right now you can only play group sounds all together at the same time, so I introduced the following playback modes:

- `blend`: default mode, play the sounds all together (like it is right now)
- `random`: play a random sound of the group
- `chain`: play the sounds in a sequence, in the order they are found in the array

It only works with `group.play()` (pausing a random sound, for example, doesn't make much sense, I think).

Other methods added:

- `group.getPlaybackMode()`
- `group.setPlaybackMode(mode)`

Thank you!
Alberto